In an air-conditioning apparatus, such as a multi-air-conditioning apparatus for a building, a refrigerant is circulated between an outdoor unit, functioning as a heat source unit, disposed outside a structure and an indoor unit disposed inside an indoor space of the structure, for example. The refrigerant rejects or receives heat, and with the heated or cooled air, heats or cools a conditioned space. As regards the refrigerant, for example, HFC (hydrofluorocarbon) is often used. An air-conditioning apparatus using a natural refrigerant, such as carbon dioxide (CO2), has also been proposed.
Furthermore, in an air-conditioning apparatus called a chiller, cooling energy or heating energy is generated in a heat source unit disposed outside a structure. Water, antifreeze, or the like is heated or cooled by a heat exchanger disposed in an outdoor unit and is carried to an indoor unit, such as a fan coil unit or a panel heater, for heating or cooling (refer to PTL 1, for example).
Moreover, an air-conditioning apparatus called a waste heat recovery chiller is constructed such that a heat source unit and each indoor unit are connected through four water pipes arranged therebetween and, for example, cooled water and heated water are simultaneously supplied so that cooling or heating can be freely selected in the indoor unit (refer to PTL 2, for example).
Furthermore, an air-conditioning apparatus is constructed such that a heat exchanger for a primary refrigerant and a secondary refrigerant is disposed near each indoor unit to carry the secondary refrigerant to the indoor unit (refer to PTL 3, for example).
Furthermore, an air-conditioning apparatus is constructed such that an outdoor unit is connected to each branching unit including a heat exchanger through two pipes to carry a secondary refrigerant to an indoor unit (refer to PTL 4, for example).